Achieving a balance between induction of protective immunity against pathogens and maintenance of self-tolerance is a central feature of the adaptive immune system. Although negative selection in the thymus removes the majority of clones that express T cell receptors (TCR) with high affinity for self-peptide MHC products, this process is incomplete. A significant fraction of mature peripheral T cells that respond to self-peptide—MHC complexes may differentiate into effector cells in the context of inflammatory stimuli (Bouneaud et al., 2000; Goldrath and Bevan, 1999; Slifka et al., 2003). Although this process is constrained by abortive or defective TCR signaling resulting in cellular elimination (AICD) or inactivation (Martin et al., 1999; Kearney et al., 1994), these cell-intrinsic mechanisms may not suffice to prevent the development of autoimmune disorders (Anderton et al., 2001; Panoutsakopoulou et al., 2001). There is increasing evidence that self tolerance may also depend on inhibitory interactions between effector T cells and regulatory or suppressive cells (Littman and Rudensky, 2010). However, regulatory T cells that are genetically programmed to inhibit development of autoantibody formation and autoimmune diseases have not been defined. Due to the severity and breath of autoimmune diseases such as lupus and rheumatoid arthritis, there is a great need for effective treatments of such diseases.